This invention generally relates to casks for holding and transporting radioactive materials, and is specifically concerned with a thermal portection shell for protecting such casks from damage in the event of a fire.
Casks for transporting radioactive materials such as the waste products produced by nuclear power plant facilities are known in the prior art. The special purpose of such casks is to ship radioactive wastes as safely as possible. Such casks may be used, for example, to ship high-level vitrified waste cannisters to a waste isolation site, or spent fuel rods to a reprocessing facility. At the present time, relatively few of such transportation casks have been manufactured and used since most of the spent fuel and other wastes generated by nuclear power plants are being stored at the reactor facilities themselves. However, the availability of such on-site storage space is steadily diminishing as an increasing amount of fuel assemblies and other wastes are loaded into the spent-fuel pools of these facilities. Additionally, the U.S. Department of Energy (D.O.E.) has been recently obligated, by the National Waste Policy Act of 1983, to move the spent fuel assemblies from the on-site storage facilities of all nuclear power plants to a federally operated nuclear waste disposal facility starting in 1998.
While the transportation casks of the prior art are generally capable of safely transporting wastes such as spent fuel to a final destination, the applicant has observed that there is considerable room for improvement, particularly in the area of fire protection. Nuclear Regulatory Commission (NRC) regulations currently require that Type B casks be capable of withstanding exposure to a fire or other source of infrared radiation which generates a temperature of 1475.degree. F. for at least thirty minutes without significant physical damage, and without the development of excessive internal temperatures and pressures. However, this particular requirement is directly at odds with the requirement that the walls of the casks be capable of conducting, at all times, the heat flux generated by the heat of decay of the radioactive materials contained therein. Accordingly, the problem cannot be solved by merely providing some sort of insulatory sheathing around the casks. While such sheathing might effectively block out excessive heat from a fire of 1475.degree. F. or higher, it would also prevent the heat of decay of the radioactive materials within the casks from dissipating into the ambient atmosphere, thereby causing the build-up of potentially excessive temperatures and pressures within the casks itself.
Various attempts have been made to solve the problem by means of an external structure around a cask that acts as a "thermal diode", readily conducting heat from the inside of the cask outwardly, but resisting the conduction of heat from the outside of the cask to its interior. However, all of the prior art attempts to solve the problem require the provision of a relatively delicate and complex structure of thermal cooling fins. Thermal bridges between the cask and the fins are either opened or closed in accordance with the effect of an external source of heat on a material within the fin structure, such as solid lithium and sodium hydroxide, or blocks of aluminum. Unfortunately, such prior art solutions suffer from a variety of shortcomings. For example, the fin structures common to each design are intricate, and require and extensive fabrication effort. Moreover, these fin structures are delicate, and apt to collapse if exposed to a large amount of mechanical shock. Such vulnerability to shock jeopardizes the ability of such prior art thermal shields to operate in a situation where a tractor-trailer transporting the cask is involved in a collision which subjects the cask to a combination of both mechanical shock and fire. Finally, each of these known thermal protection shields are an integral part of the casks which they attempt to protect. Accordingly, none of these shields is easily adaptable for use on a prior art cask which lacks the thermal protection now required by the NRC.
Clearly, what is needed is a thermal protection shield which is simple and rugged in construction, and not apt to lose its shielding properties when exposed to a substantial mechanical shock. Ideally, such a shield should further be easily machinable out of common materials so as to minimize the cost of fabrication. Finally, it would be desirable if the thermal shield could easily be used on casks already in existence.